Threshold detector for incident radiation

ABSTRACT

A resistance bridge, having one diagonal connected across a source of direct current, includes in its other diagonal the base/emitter circuit of an input transistor controlling a switching transistor of the opposite conductivity-type. The latter transistor controls a relay which operates when a photoresistor in one of the arms of the bridge is illuminated; a regenerative feedback connection from the output of the switching transistor exerts a toggle effect by modifying the bias of the input transistor to vary the sensitivity of the detector in a sense tending to maintain the relay in either its operated or its unoperated state.

United States Patent Del Zotto [54] THRESHOLD DETECTOR FOR INQIDENT RADIATION [72] Inventor: Giorgio Del Zotto, Milan, Italy [73] Assignee: Ates Componeti Elettronlel S.p.A., Milan,

Italy [22] Filed: Apr. 28, 1970 [21] Appl.No.: 32,658

301 rm Application Priority um May 9, 1969 [52] US. Cl ..250/210, 317/124, 315/159 [51] 1nt.Cl.. 101] 39/12 [58] FleldolSearch ..250/210,2l4;315/l59, 158,

[56] References Cited UNITED STATES PATENTS Campbell "250/2 1 0 X Italy 16618 A169 1 [451 Jan. 18,1972

3,458,770 7/1969 Denger ..3l5/l59X 3,094,617 6/1963 HumphriesetaL. .,..2$0/210X 3,273,752 9/1966 Horeczlty ..250/210X Primary Examiner-Walter Stolwein Attorney-4hr] F. Ross [57] ABSTRACT A resistance bridge, having one diagonal connected across a source of direct current, includes in its other diagonal the base/emitter circuit of an input transistor controlling a switching transistor of the opposite conductivity-type. The latter transistor controls a relay which operates when a photoresistor in one of the arms of the bridge is illuminated; a regenerative feedback connection from the output of the switching transistor exerts a toggle effect by modifying the bias of the input transistor to vary the sensitivity of the detector in a sense tending to maintain the relay in either its operated or its unoperated state.

9991mm Drai s PATENTED JAN18I972- 34636357 Giorgio Del Zorro INVENTOR.

Attorney light or set of lightsto be .tumed on automaticallyat dusk and to be switched off automatically atdawn. Typical for such field of application are the courtesy lights of anautomotive vehicle in jurisdictions requiring the illumination of the contours of anautomobile parked in the street. Other instances are porch lights and indoor illumination of dwellings whose owners wish to create the impression of nighttime'occupancy. 1

Theme of .a radiation-sensitive resistance element, such as a photodiode, readily suggests itself for this purpose. Onesdisadvantage of conventional circuitry .using photodiodesis their thermal sensitivity which causes variations in theirluminous threshold with changes in ambient temperaturexSuchthermal sensitivity also manifests itself with similar disadvantageous effect in transistors used to amplify the photodiode output.

An object of my present invention,.therefore, is toprovide an improved radiationdetector of the character referred to whichv is substantially insensitive to variations in ambient temperature.

The conductivity .of photodiodes as well as transistors is also affected by changes in the supply voltage. When the power supply isthe'battery of an automotive vehicle, for example, the supply voltage may vary considerably from one day toathe .next. Thus, another object of my inventionis to provide a radiation detector of this description which is substantially insensitiveto moderate fluctuations in the supply voltage.

A further object of my invention is to provide a system wherein theeffectiveresponse threshold of a photoresistor or equivalent radiation-sensitive element can be modified so as to be differentfor the cut-in and the cutout, thereby creating a toggle effect which prevents any untimely switching of the load byminor variationsin the level of incident radiation (e;g., illumination of the sensor by the beam of a passing vehicle: at night).

lnmycopending application Ser. No. 879,46 l filed Nov. l969,l havedisclosed a radiation-sensitive'threshold detector with a resistive bridge networkhaving one diagonal connected across a direct current source and including in its other diagonal a photodiode in series with the base/emitter circuits of two-transistors of opposite conductivitytypes,i.e.,*an NPN- transistor and a PNP-transistor. The second transistor "included in this bridge networkhas itscollector. connected inseries with an input circuit of a transistorizied amplifiercomprisingtwo transistor stages whose conductivity type is'the same as that of the firstltransistor.(e.g., NPN).'The load circuit includes a relay serving for the alternate energization and .deenergization of a controlled device (e;.g., a lamp) in response to variationsin the level of radiation? incidentupon .the sensor. In its turn, the relay controls a switch for modifying the bias of :one-ofthe transistors formingpartofthe bridge network for therpurpose of feeding back a signal tending to stabilize the relay in its operated or unoperated condition. Owing to the resulting toggle effect, the control circuit forthe relay responds to different threshold levels of the-sensor output in switching from oneoperating condition to the other. 7

ln contradistinction to mysprior systein','my present invention replaces the relay-controlledutoggle switchby arpurely electronic circuit, i.e., by :a iregenerative feedback path between the output andJthe input of anfamplifier preferably consisting of two transistor stages. Thefirst of thesestages, referred to hereinafter. as the .control transistor, 'has itslinput connected across the outputdiagonal of a resistance bridge, similar to .the aforedescribed network, which also includes a photoresistor in one of its resistive arms. The junction between this photoresistor and a series U resistor included'in that bridge arm is connected to an output terminal of the amplifier, such as the collector of the second stagereferred to hereinafter asthe switching.transistor,fQrJmodifying'theEbias of thecontrol transistor in response to a change in the conductivity of the switchingtransistor. This, my improved system entails a substantial simplification drastically reducing the number of transistorsrequired. 1

The invention will be described in greater detail hereinafter with reference to the accompanying drawing the sole FIGURE of which shows a circuit diagram of a representative embodiment.

The system shown in the drawing comprises apair of bus bars ll, 12 connected across a source of direct current V,,,,,

.such as an automotive battery havinga ground negative terminal tied to bus bar 12 and a positiveterminal tied to bus bar 11. Between these terminals lies a load in the form of a lamp L in series'with a ballast resister'R, and an annature and front contact of a relay RL.

A resistance bridge constituted by four resistors R,-'R,, R,,

R has-its input diagonal connectedbetween bus bars ll, 12

and further includes aphotoresistor FR in-series with bridge arm R The output diagonal of the bridgeis connected across the base/emitter circuit of a first or control transistor Q. 'whosecollector is tied .to'the base of a second or switching transistor 0,; the two transistors are of opposite conductive types, .i.e.,of PNP-type in the case of the former and NPN- type in the case of the latter, and together form a two-stage signal amplifier. The output circuit of transistor 0 is in series with the winding of relayRL, with its emitter grounded at bus bar l2and its collector connected to bus bar 11 by way of that winding. The base of transistor O is biased by.a resistor R, while afurther resistorR lies'between the junction A of resistor-k -with photoresistor PR. and the junction B of the relay winding with the collector of transistor 0,. I

Resistor R forms part of a regenerative feedback path from thezoutput to .thednput of .the two-stage amplifier 0,, 0 Resistors-R and R are shown to be adjustable.

ln daylight, i.e., when photoresistor FR is strongly illuminated and its resistance is relatively low, the base of transistor Q is driven positive with. reference to the emitter whose potential is determined by thevoltagedivider consistingof resistors R and R Under these conditions the transistor .Q,-.is cutoff, as-is the transistor Q whose base is then effectivelygrounded. Relay RLis unoperated so that lamp L is not 'lit. Thevinput voltage V (at-point A) and the output voltage becomeconductive until-the effect of the positive potential at point .B .has been overcome. When this happens, both transistors Q Q: are turned on and the consequent lowering of output voltage V exerts a toggle reffect accelerating the saturation of the amplifier. This results in an alternate stable :condition in which even anillumination of photoresistor FR by .a light source of limited intensity will not cut off the transistor Q to release 'the relayRLySuch release will occur on'lyupona sufi'icient irradiation ofvthe photoresistor to overcome the virtual grounding of point B by the saturated transistor 0 "I It willcbe apparent'that the cut-in threshold can be varied by the adjustment of resistor R and that the cutout'threshold can be similarly altered by the adjustment' of resistor R with the resistance of the relay winding (together with any resistor in series .therewith) high compared to ,that of resistor R,, the setting. of resistor'R will have but a negligible effect upon the first-mentioned threshold.

Thus, the two variable resistors R, R can be used .to change at will the hysteresis or toggle effect that prevents a switchover from one stable condition to the other upon a change in illumination by less than apredetermined magnitude which may be different for the two switching conditions.

The change. Avm the input voltage of the amplifier is proportional .tothe voltage swingAV of-its output by a feedback The voltageswingAV, is, of course, the difference between the values of V with transistor Q either cut oh or saturated.

I have found, in practical tests, that such a system may be insensitive to substantial variations in supply voltage, e.g., between 8 and l6 volts, as well as in ambient temperature, e.g., between 20 and +75 C. The upper and lower thresholds of luminous intensity may be at 400 and l lux, respectively, with a drift of less than 1.5 lux/"C in response to temperature changes and less than lux/V in response to voltage fluctuations.

Lamp L is representative of any light source controlled in response to ambient luminosity, e.g., the courtesy lights of a parked automotive vehicle.

I claim:

1. A circuit arrangement for detecting incident radiation, comprising a source of direct current; a resistance bridge having one diagonal connected across said source; a radiationsensitive resistance element and resistance means serially connected in an arm of said bridge; a signal amplifier having an input connected across the other diagonal of said bridge; a load circuit including relay means connected to the output of said amplifier for alternate energization and deenergization in response to variations in the level of radiation incident upon said resistance element; and regenerative feedback path constituted by a conductive connection extending from said output to a junction between said resistance means and said resistance element.

2. A circuit arrangement as defined in claim I wherein said feedback path includes an adjustable resistor.

3. A circuit arrangement as defined in claim I wherein said relay means has a winding in series with a pair of output electrodes of said amplifier, said feedback path originating at a junction of one of said output electrodes with said winding.

4. A circuit arrangement as defined in claim 3 wherein said amplifier comprises a first transistor stage of one conductivity type and a second transistor stage of opposite conductivity type, said one of said output electrodes being a collector of said second stage.

5. A circuit arrangement as defined in claim 4 wherein said first stage has an emitter connected to a junction of two other arms of said bridge, one of said other arms including an adjustable resistor.

6. A circuit arrangement as defined in claim 4 wherein said load circuit forms a resistive connection including said wind ing between said collector and a terminal of said source, the resistance of said resistive connection being high compared with that of said resistance means. 

1. A circuit arrangement for detecting incident radiation, comprising a source of direct current; a resistance bridge having one diagonal connected across said source; a radiation-sensitive resistance element and resistance means serially connected in an arm of said bridge; a signal amplifier having an input connected across the other diagonal of said bridge; a load circuit including relay means connected to the output of said amplifier for alternate energization and deenergization in response to variations in the level of radiation incident upon said resistance element; and regenerative feedback path constituted by a conductive connection extending from said output to a junction between said resistance means and said resistance element.
 2. A circuit arrangement as defined in claim 1 wherein said feedback path includes an adjustable resistor.
 3. A circuit arrangement as defined in claim 1 wherein said relay means has a winding in series with a pair of output electrodes of said amplifier, said feedback path originating at a junction of one of said output electrodes with said winding.
 4. A circuit arrangement as defined in claim 3 wherein said amplifier comprises a first transistor stage of one conductivity type and a second transistor stage of opposite conductivity type, said one of said output electrodes being a collector of said second stage.
 5. A circuit arrangement as defined in claim 4 wherein said first stage has an emitter connected to a junction of two other arms of said bridge, one of said other arms including an adjustable resistor.
 6. A circuit arrangement as defined in claim 4 wherein said load circuit forms a resistive connection including said winding between said collector and a terminal of said source, the resistance of said resistive connection being high compared with that of said resistance means. 